The long term objective of these studies is to define the role of the enhancer factor PEA3 in the developmental regulation of mammalian gene expression. PEA3 was first identified in our laboratory and is important for the function of the polyomavirus enhancer, which activates both viral transcription and viral DNA replication. My preliminary studies indicate that PEA3 binding sites are present in the regulatory sequences of many proto-oncogenes and developmentally regulated genes. To determine the role of PEA3 in regulatory proto-oncogenes and other genes during embryonic development, the specific aims of this proposal are to: (a) purify PEA3 binding activity, prepare monoclonal antibodies against PEA3 and determine the sequence specificity of PEA3 binding; (b) clone and characterize the gene(s) encoding PEA3; (c) analyze the regulatory sequences responsible for regulation of PEA3 expression; (d) express sense and anti-sense PEA3 RNAs in differentiating F9 cells to determine the role of PEA3 in the expression of protooncogenes and developmentally regulated genes which have PEA3 binding sites in their regulatory regions; (e) analyze PEA3 expression in mouse embryos at the RNA level by in situ hybridization. and at the protein level by in situ immunocytochemistry.